There is an important commercial need to obtain a color proof that will accurately represent at least the details and color tone scale of the image before a printing press run is made. In many cases, it is also desirable that the color proof accurately represents the image quality and halftone pattern of the prints obtained on the printing press. In the sequence of operations necessary to produce an ink-printed, full-color picture, a proof is also required to check the accuracy of the color separation data from which the final three or more printing plates or cylinders are made.
Multiple dye-donors are generally used to obtain a range of colors in the proof as is described in U.S. Pat. No. 5,126,760 (DeBoer). For a full-color proof, four colors (cyan, magenta, yellow and black) are normally used. Although a wide gamut of printing ink colors can be matched by just a few dye-donor elements, there are certain types of inks and pigments used in the printing industry that cannot be matched by any combination of dyes. Notable among these types of inks and pigments are the metallics, white and opaque spot colorants.
A continuing trend in the printing industry is the increasing use of specialty inks such as metallic specialty inks. Metallic specialty inks can increase color gamut, provide signature colors, and generate special effects. In the advertising and packaging marketplaces this translates into greater appeal and recognition. Although the particular color can be approximated by standard process color inks, the specular reflectivity characteristic that gives metallic colors their special appeal requires the use of metal flakes in the metallic specialty ink formulation.
In response to the increased use of metallic specialty inks, a single layer, metallic donor formulated using an aluminum flake was developed for the KODAK APPROVAL XP digital color proofing system disclosed in U.S. Pat. No. 6,197,474 (Niemeyer, et al.).
The KODAK APPROVAL XP system uses successive dye containing donor films placed against an intermediate receiver film and exposed through the base of the donor films with an 830 nm laser diode array. Because the KODAK APPROVAL XP system is capable of printing multiple colors at variable density at the same location, multiple metallic dye-donor films need not be developed. Gold, bronze, copper, and the host of metallic reds, greens and blues can be obtained by overprinting brilliant silver. The multicolor dye image, along with the top layer of the intermediate receiver film, is laminated to a final receiver.
The mechanism of dye transfer in the KODAK APPROVAL XP digital color proofing system is volatilization. This mechanism is not well suited, however, for the transfer of non-volatile aluminum flakes and does not produce the resolution necessary for accurate halftone color proofs.
The use of a two-layer film in a laser ablative process is described in “Metallic Donor for Direct Digital Halftone Proofing”, IS&T's NIP18: 2002 International Conference on Digital Printing Technologies, David A. Niemeyer, pp. 718-21. A two-layer film for use in an ablative process in which a metallic flake layer overlays an infrared, radiation sensitive, propellant layer is reported. Gasification of the propellant layer upon exposure by an 830 nm laser diode array provides the motive force to transfer the metallic flake layer from the donor to the receiver. Specific polymers are selected which decompose upon exposure to heat to rapidly generate a gas. Examples of other laser ablative systems may be found in U.S. Pat. No. 5,516,622 (Savini, et al.); U.S. Pat. No. 5,518,861 (Coveleski, et al.); U.S. Pat. No. 5,326,619 (Dower, et al.); U.S. Pat. No. 5,308,737 (Bills, et al.); U.S. Pat. No. 5,278,023 (Bills, et al.); U.S. Pat. No. 5,256,506 (Ellis, et al.); U.S. Pat. No. 5,171,650 (Ellis, et al.); U.S. Pat. No. 5,156,938 (Foley, et al.); and U.S. Pat. No. 3,962,513 (Eames).
There is a problem with this laser ablative transfer mechanism, however, as the use of propulsive forces introduce a tendency for the colorant to scatter and produce less well defined dots made of many fragments. Attempts have been made to produce more well defined dots using an ablative system as described in U.S. Pat. No. 5,156,938 (Foley) and U.S. Pat. No. 5,171,650 (Ellis); however, whether single layer or dual layer, such systems do not produce contract-quality images. Further, decomposition of the polymers selected to decompose upon exposure to heat to rapidly generate a gas lends to discoloration of the halftone color proof. Therefore, this process lacks the necessary resolution to produce an accurate halftone color proof.
Alternative mass transfer systems include a melt mechanism. In a melt mechanism, the colorant and associated binder materials transfer in a molten or semi-molten state (melt-stick transfer) to a receptor upon exposure to the radiation source. There is essentially 0% or 100% transfer of colorant depending on whether the applied energy exceeds a certain threshold. Examples of these types of systems may be found in JP 63-319192 (Seiichiro); JP 69-319192 (Naoji, et al.); EP 530 018 (Hitomi); EP 602 893 (Patel, et al.); EP 675 003 (Patel); EP 745 489 (Patel, et al.); U.S. Pat. No. 5,501,937 (Matsumoto, et al.); U.S. Pat. No. 5,401,606 (Reardon, et al.) and U.S. Pat. No. 5,019,549 (Kellogg, et al.).
In contrast to ablative systems, melt systems can in principle form more well-defined dots and sharper edges to achieve more reproducible and accurate colors, however, the system involves other disadvantages. Many of the known laser-induced melt transfer systems employ one or more waxes as binder materials. The use of waxes results in a transfer layer that melts sharply to a highly fluid state at moderately elevated temperatures, and hence gives a higher sensitivity; however, such systems arc prone to image spread as a result of wicking or uncontrolled flow of the molten transfer material. Furthermore, because the laser absorber is normally transferred along with the desired colorant, the final image may lack the accuracy of color rendition required for high quality proofing purposes. Others have attempted to increase the sensitivity by adding plasticizers (U.S. Pat. No. 5,401,606 (Reardon)), which lower the melt viscosity and increase the flow; however, the plasticizers soften the films such that they become receptive to impressions and blocking.
Thus, there is still a need for a laser-induced thermal transfer system that provides a halftone image incorporating metallic flakes in the form of discrete dots having well-defined, generally continuous edges that are relatively sharp with respect to density or edge definition.